A system rule named "Forbid Sites" will be listed. Choose "Edit", and you will be shown a textbox. In the textbox, one per line, list the websites or web pages that you don't want to visit.

These rules only apply to pages that are being considered for inclusion, not to files already indexed. After editing the Forbid Sites rule, you may rebuild your index to make the new rule apply to current content.

"How to forbid pages that you don't control"
http://www.xav.com/scripts/search/help/1047.html